Put the weight box on the right side in the trunk as far back as you can get it, is the battery in the trunk now? If not put it there now also, in front of the weight box so you can remove the battery easily as needed instead of fighting the weight box. Most of the NHRA class racers I know use lead weights in 5 lb. increments in the box to make the mininum car weight for class. If you need to add permenent weight it has to be bolted to the frame or a frame member like a cross memeber to be NHRA legal, check the rule book to do that the legal way
I used a 45 lb piece of steel bolted to the rear shock cross memeber on my stocker, a heavy spare tire(big radial) and wheel and then added weight to the weight box to make the car legal weight to run the next lower class, from A/SA to B/SA. Added weight on the rear usually helps make the car hook up better, if the car is a factory stocker and has not been massaged to get better weight distibution.
Good luck, keep us posted on your changes and results